A. Field of Invention
This invention pertains to an external defibrillator arranged and constructed to provide anti-tachyarrhythmia therapy to a patient on demand. In particular, an automatic external cardioverter/defibrillator is described which has several operational modes including a fully automatic mode in which shocks are delivered without any manual intervention, an advisory mode and a manual mode. Moreover, the invention pertains to a defibrillator with an integral tachyarrhythmia detector which detects an abnormal heart beat and determines whether this abnormal heart beat is amenable to shock therapy.
B. Description of the Prior Art
Defibrillators are devices which apply electric therapy to cardiac patients having an abnormally high heart rhythm. Two kinds of defibrillators are presently available: internal defibrillators which are implanted subcutaneously in a patient together with leads extending through the veins into the cardiac chambers, and external defibrillators which are attached (usually temporarily) to the patient. External defibrillators are used in most instances in case of an emergency, for example, when a patient has either suffered cardiac arrest or where a cardiac arrest is imminent. Typically therefore external defibrillators are manual devices which must be triggered by a physician or other trained personal. Internal or implantable defibrillators (and cardioverters) are implanted as a permanent solution for patients having specific well defined cardiac deficiencies which cannot be treated successfully by other means. They generally operate in an automatic mode.
However, there are some instances where an external defibrillator would be very advantageous which could be operated in both automatic and manual modes. For example, presently, it is well known that after a cardiac episode, such as a sudden cardiac arrest, many patients frequently suffer a second episode of similar nature. Therefore, cardiac patients are kept in a hospital under observation. While in the hospital, the patient is attached to a monitor which indicates the patient's heart rate, temperature, respiration rate and other vital signs. Many monitors are provided with an alarm function which is activated when these vital signs fall outside a nominal range. The monitor then generates an audible and visual signal at the bed side of the patient and/or at the remote location such as a nurse station. However, if a cardiac episode does occur, the attending staff has to examine the patient to determine that the patient needs electrical therapy, and then set up and use a manual defibrillator. All these steps are inherently time-consuming.
Some attempt has been made to overcome some of these problems. For example, some external defibrillators are available which can verify that a patient is suffering from ventricular fibrillation VF and prompt an attendant to activate the defibrillator for the delivery of therapy. However, the algorithms used by these defibrillators to detect VF are very limited. For example, some of defibrillators utilize an algorithm in which the electrocardiogram ECG can be verified only while the patient is unconscious, has no pulse and does not breath. Obviously, these algorithms are not satisfactory since it is important to detect when those conditions happen and apply therapy as soon as those conditions are detected.
Commonly-owned U.S. Pat. No. 5,474,574 discloses an external defibrillator. Commonly-owned U.S. Pat. No. 4,576,170 discloses an external defibrillator that can be worn by a patient.